Deep Ultraviolet Light Emitting Diode

ABSTRACT

A method of fabricating a light emitting diode, which includes an n-type contact layer and a light generating structure adjacent to the n-type contact layer, is provided. The light generating structure includes a set of quantum wells. The contact layer and light generating structure can be configured so that a difference between an energy of the n-type contact layer and an electron ground state energy of a quantum well is greater than an energy of a polar optical phonon in a material of the light generating structure. Additionally, the light generating structure can be configured so that its width is comparable to a mean free path for emission of a polar optical phonon by an electron injected into the light generating structure.

REFERENCE TO PRIOR APPLICATIONS

The current application is a continuation of U.S. Utility applicationSer. No. 14/514,586, which was filed on 15 Oct. 2014, and which is acontinuation of U.S. Utility application Ser. No. 13/803,681, which wasfiled on 14 Mar. 2013, which claims the benefit of U.S. ProvisionalApplication No. 61/610,671, which was filed on 14 Mar. 2012, and whichis a continuation-in-part of U.S. Utility application Ser. No.13/161,961, which was filed on 16 Jun. 2011, which claims the benefit ofU.S. Provisional Application No. 61/356,484, which was filed on 18 Jun.2010, all of which are hereby incorporated by reference.

GOVERNMENT LICENSE RIGHTS

The U.S. Government has a paid-up license in this invention and theright in limited circumstances to require the patent owner to licenseothers on reasonable terms as provided for by the terms of SBIR Phase IIGrant No. IIP-0956746 awarded by the National Science Foundation.

TECHNICAL FIELD

The disclosure relates generally to nitride-based heterostructures, andmore particularly, to an improved ultraviolet light emittingnitride-based heterostructure.

BACKGROUND ART

Emerging deep ultraviolet light emitting diodes (DUV LEDs) cover theultraviolet (UV) range down to 210 nanometers (nm), and provide outputpowers already sufficient for many applications. Additionally, thesedevices have high modulation frequencies, low noise, flexible formfactor and spectral and space power distribution, high internal quantumefficiency, and a potential to achieve high wall plug efficiency. Forexample, photoluminescence (PL) studies and ray tracing calculationsshow that the achieved internal quantum efficiency for a 280 nm DUV LEDmay be quite high, e.g., between fifteen and seventy percent.

However, external quantum efficiency and wall plug efficiency of typicalDUV LEDs is below three percent, with the highest efficiencies for 280nm LEDs and lower efficiencies for LEDs emitting ultraviolet lighthaving shorter wavelengths. Some reasons for the lower external and wallplug efficiencies include very low light extraction efficiency due tointernal reflection from the sapphire substrate and sapphire/airinterface, and strong absorption in the top low aluminum (AD-contentp-type aluminum gallium nitride (AlGaN) and p-type gallium nitride (GaN)layers. The efficiency of the LEDs is further reduced at higher currentsand/or generated powers.

In UV LEDs emitting ultraviolet light having a shorter wavelength, theinternal quantum efficiency also drops due to materials problemsresulting from growth of AlGaN structures with high Al content. Suchgrowth, among other things, is complicated by the low mobility of Aladatoms, which can result in inhomogeneous Al composition and lateralphase separation, as well as high density of threading dislocations andpoint defects.

SUMMARY OF THE INVENTION

Aspects of the invention provide a method of fabricating a lightemitting diode, which includes an n-type contact layer and a lightgenerating structure adjacent to the n-type contact layer. The lightgenerating structure includes a set of quantum wells. The contact layerand light generating structure can be configured so that a differencebetween an energy of the n-type contact layer and an electron groundstate energy of a quantum well is greater than an energy of a polaroptical phonon in a material of the light generating structure.Additionally, the light generating structure can be configured so thatits width is comparable to a mean free path for emission of a polaroptical phonon by an electron injected into the light generatingstructure. The diode can include a blocking layer, which is configuredso that a difference between an energy of the blocking layer and theelectron ground state energy of a quantum well is greater than theenergy of the polar optical phonon in the material of the lightgenerating structure. The diode can include a composite contact,including an adhesion layer, which is at least partially transparent tolight generated by the light generating structure and a reflecting metallayer configured to reflect at least a portion of the light generated bythe light generating structure.

A first aspect of the invention provides a method of fabricating a lightemitting heterostructure, the method comprising: creating a design forthe light emitting heterostructure based on a set of attributes of apolar optical phonon, wherein the light emitting heterostructureincludes an n-type contact layer and a light generating structureincluding a plurality of quantum wells and having a first side adjacentto the n-type contact layer, and wherein the creating includes:selecting materials for the n-type contact layer and a quantum well inthe plurality of quantum wells based on an energy of the polar opticalphonon in a material of the light generating structure, wherein adifference between a conduction band edge energy of the n-type contactlayer and an electron ground state energy of the quantum well is greaterthan the energy of the polar optical phonon; and selecting a targetwidth of the light generating structure based on a mean free path foremission of a polar optical phonon by an electron injected into thelight generating structure, wherein the target width is comparable tothe mean free path; and fabricating the light emitting heterostructureaccording to the design.

A second aspect of the invention provides a method of fabricating alight emitting heterostructure, the method comprising: creating a designfor the light emitting heterostructure based on a set of attributes of apolar optical phonon, wherein the light emitting heterostructureincludes an n-type contact layer, a light generating structure includinga plurality of quantum wells and having a first side adjacent to then-type contact layer, and a blocking layer located on a second side ofthe light generating structure opposite the first side, and wherein thecreating includes: selecting materials for the blocking layer and aquantum well in the plurality of quantum wells based on an energy of thepolar optical phonon in a material of the light generating structure,wherein a difference between a conduction band edge energy of theblocking layer and an electron ground state energy of the quantum wellis greater than the energy of the polar optical phonon; and selecting atarget width of the light generating structure based on a mean free pathfor emission of a polar optical phonon by an electron injected into thelight generating structure, wherein the target width is comparable tothe mean free path; and fabricating the light emitting heterostructureaccording to the design.

A third aspect of the invention provides a method of fabricating a lightemitting device, the method comprising: forming an n-type contact layer;forming a light generating structure having a first side adjacent to then-type contact layer, the light generating structure including: a set ofquantum wells, wherein a difference between a conduction band edgeenergy of the n-type contact layer and an electron ground state energyof a quantum well in the set of quantum wells is greater than an energyof a polar optical phonon in a material of the light generatingstructure; and a set of barriers interlaced with set of quantum wells,the set of barriers including a first barrier immediately adjacent to afirst quantum well in the set of quantum wells and the first side of thelight generating structure, wherein a thickness of the first barrier issufficient to accelerate an injected electron in an electric field toreach the energy of the polar optical phonon with respect to theelectron ground state energy in the first quantum well, and wherein athickness of a remainder of the light generating structure exceeds amean free path for emission of a polar optical phonon by an electroninjected into the light generating structure.

Additional aspects of the invention provide methods of designing and/orfabricating the heterostructures and devices shown and described herein,as well as methods of designing and/or fabricating circuits includingsuch devices, and the resulting circuits. The illustrative aspects ofthe invention are designed to solve one or more of the problems hereindescribed and/or one or more other problems not discussed.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other features of the disclosure will be more readilyunderstood from the following detailed description of the variousaspects of the invention taken in conjunction with the accompanyingdrawings that depict various aspects of the invention.

FIG. 1 shows an illustrative band diagram of a deep UV light emittingheterostructure including an energy tub according to a previoussolution.

FIG. 2 shows a band diagram of an illustrative light emittingheterostructure according to an embodiment.

FIG. 3 shows a band diagram for an illustrative light emittingheterostructure according to another embodiment.

FIG. 4 shows a band diagram for an illustrative light emittingheterostructure according to yet another embodiment.

FIG. 5 shows a band diagram for an illustrative light emittingheterostructure according to still another embodiment.

FIG. 6 shows an illustrative heterostructure for a light emitting diodeaccording to an embodiment.

FIG. 7 shows reflection coefficients of different coatings forillustrative reflective contacts.

FIGS. 8A-8D show illustrative LED configurations with composite contactsaccording to embodiments.

FIG. 9 shows a chart comparing illustrative transmission spectra ofconventional and transparent 340 nanometer DUV LEDs structures.

FIG. 10 shows a chart illustrating an illustrative performanceimprovement of a 340 nm DUV LED structure with a reflecting contact.

FIG. 11 shows an illustrative configuration for a flip chip LEDaccording to an embodiment.

FIG. 12 shows a dependence of the absorption coefficient on thewavelength for various aluminum molar fractions (x) of anAl_(x)Ga_(1-x)N alloy according to an embodiment.

FIG. 13 shows an illustrative chart for selecting an aluminum content ofan AlGaN alloy to maintain a target transparency for a correspondingemitted wavelength according to an embodiment.

FIG. 14 shows an illustrative flow diagram for fabricating a circuitaccording to an embodiment.

It is noted that the drawings may not be to scale. The drawings areintended to depict only typical aspects of the invention, and thereforeshould not be considered as limiting the scope of the invention. In thedrawings, like numbering represents like elements between the drawings.

DETAILED DESCRIPTION OF THE INVENTION

As indicated above, aspects of the invention provide a light emittingdiode, which includes an n-type contact layer and a light generatingstructure adjacent to the n-type contact layer. The light generatingstructure includes a set of quantum wells. The contact layer and lightgenerating structure can be configured so that a difference between anenergy of the n-type contact layer and an electron ground state energyof a quantum well is greater than an energy of a polar optical phonon ina material of the light generating structure. Additionally, the lightgenerating structure can be configured so that its width is comparableto a mean free path for emission of a polar optical phonon by anelectron injected into the light generating structure. The diode caninclude a blocking layer, which is configured so that a differencebetween an energy of the blocking layer and the electron ground stateenergy of a quantum well is greater than the energy of the polar opticalphonon in the material of the light generating structure. The diode caninclude a composite contact, including an adhesion layer, which is atleast partially transparent to light generated by the light generatingstructure and a reflecting metal layer configured to reflect at least aportion of the light generated by the light generating structure. Asused herein, unless otherwise noted, the term “set” means one or more(i.e., at least one) and the phrase “any solution” means any now knownor later developed solution. Furthermore, as used herein, it isunderstood that the term “light” includes electromagnetic radiation ofany wavelength, whether within the visible spectrum or outside of thevisible spectrum.

Turning to the drawings, FIG. 1 shows an illustrative band diagram of adeep UV light emitting heterostructure 2 including an energy tub 4according to a previous solution. In particular, a light generatingmultiple quantum well (MQW) structure 6 of the heterostructure 2 isconfined to the energy tub 4. However, the inventors have found thatsuch a band diagram can be difficult to implement for short wavelengthstructures, in which the Al molar fraction is very high, e.g., greaterthan fifty percent.

FIG. 2 shows a band diagram of an illustrative light emittingheterostructure 10 according to an embodiment. In this case, theheterostructure 10 includes a light generating structure 12 and an atleast partially transparent (e.g., semi-transparent or transparent)injector cladding layer 14 adjacent to the light generating structure12. As illustrated, the light generating structure 12 can includeinterlaced sets of barriers (higher energy in the band diagram) andquantum wells (lower energy in the band diagram). To this extent, eachquantum well in the light generating structure 12 has one or moreadjacent barriers and each barrier in the light generating structure 12has one or more adjacent quantum wells. In heterostructure 10, an energydifference 16 (e.g., band offset) between an energy of an n-type contactlayer 18 and an electron ground state energy level in a quantum well inthe light generating structure 12 is slightly larger than the energy ofa polar optical phonon, E_(OPT-PHONON), within a material of the lightgenerating structure 12. In an embodiment, the energy difference 16exceeds the energy of the polar optical phonon by approximately thermalenergy, which is approximately twenty-six milli-electron Volts (meV) atroom temperature.

Furthermore, a total width 13 of the light generating structure 12 canbe selected to be comparable to a mean free path for emission of a polaroptical phonon by an electron injected into the light generatingstructure 12. In an embodiment, the width 13 of the light generatingstructure 12 is configured to be slightly larger than the mean freepath, e.g., exceeding the mean free path by less than approximately tenpercent. In an embodiment, the width 13 of the light generatingstructure exceeds the mean free path by less than approximately fivepercent. However, it is understood that in other embodiments, the width13 of the light generating structure can exceed the mean free path foremission of the polar optical phonon by greater than ten percent. Theillustrative design of heterostructure 10 can achieve one or more of:enhanced transitions of the injected electrons into multiple quantumwells; confinement of the injected electrons in the quantum wells; andimproved uniformity of the electron distribution between the multiplequantum wells.

The various layers of heterostructure 10 can be formed using anyappropriate material compositions. In an illustrative embodiment, thelayers 12, 14, 18 are formed using differing wide band gap semiconductormaterials, such as differing group III nitride material compositions.Group III nitride materials comprise one or more group III elements(e.g., boron (B), aluminum (Al), gallium (Ga), and indium (In)) andnitrogen (N), such that B_(W)Al_(X)Ga_(Y)In_(Z)N, where 0≦W, X, Y, Z≦1,and W+X+Y+Z=1. Illustrative group III nitride materials include AlN,GaN, InN, BN, AlGaN, AlInN, AlBN, AlGaInN, AlGaBN, AlInBN, and AlGaInBNwith any molar fraction of group III elements. In an embodiment, thematerials include any combination of: AlN, GaN, InN, and/or BN alloys.

In an embodiment, cladding layer 14 comprises an at least partiallytransparent magnesium (Mg)-doped AlGaN/AlGaN short period superlatticestructure (SPSL). In another embodiment, the n-type contact layer 18comprises a cladding layer formed of a short period superlattice, suchas an AlGaN SPSL, which is at least partially transparent to radiationgenerated by the light generating structure 12.

FIG. 3 shows a band diagram for an illustrative light emittingheterostructure 20 according to another embodiment. In heterostructure20, a blocking layer 22 is also included adjacent to the cladding layer14. In an embodiment, the blocking layer 22 can comprise a group IIInitride material having a graded or modulated aluminum composition alonga width of the blocking layer 22. In another embodiment, the blockinglayer 22 can comprise a superlattice structure, which can enable animproved materials quality in the heterostructure 20. Blocking layer 22can be configured as an electron blocking layer and/or as a claddinglayer using any solution.

FIG. 4 shows a band diagram for an illustrative light emittingheterostructure 30 according to yet another embodiment. Inheterostructure 30, a thickness 32 (as measured in the direction oftravel for the electrons) of a first barrier 15 in the light generatingstructure 12 is selected to be sufficient to accelerate electronsinjected into the light generating structure 12 from the n-type contact18 to reach an energy of a polar optical phonon, E_(OPT-PHONON), withrespect to the energy states in the quantum wells. Furthermore, athickness 34 of a remainder of the light generating structure 12 can beselected to be comparable to (e.g., slightly exceed) the mean free pathfor the emission of polar optical phonons by electrons.

FIG. 5 shows a band diagram for an illustrative light emittingheterostructure 40 according to still another embodiment. Inheterostructure 40, an energy difference 44 (e.g., band offset) betweenan energy of a p-type blocking layer 42 and an electron ground stateenergy in a quantum well within the light generating structure 12 isslightly larger than the energy of the polar optical phonon,E_(OPT-PHONON), in the material of the light generating structure 12. Inan embodiment, the energy difference exceeds the energy of the polaroptical phonon by approximately thermal energy. Blocking layer 42 can beconfigured as an electron blocking layer and/or as a cladding layerusing any solution.

FIG. 6 shows an illustrative heterostructure 50 for a light emittingdiode (LED) according to an embodiment. As illustrated, theheterostructure 50 can include a substrate 52, an n-type contact 54, alight generating structure 56, and a p-type contact 58. In anembodiment, the substrate 52 and n-type contact 54 are at leastpartially transparent to the light generated by the light generatingstructure 56, thereby enabling extraction of light generated by thelight generating structure 56 out of the heterostructure 50 through thetransparent substrate 52. Furthermore, the heterostructure 50 caninclude a distributed semiconductor heterostructure Bragg reflector(DBR) structure 60 on an opposing side of the light generating structure56 than a transparent side of the heterostructure 50 (e.g., thetransparent substrate 52). The DBR structure 60 can be configured toreflect additional light generated by the light generating structure 56out of the transparent substrate 52 than would otherwise be provided.Additionally, the heterostructure 50 can include an electron blockinglayer 61 located between the DBR structure 60 and the light generatingstructure 56, which can suppress residual electron overflow from then-type contact 54 to the p-type contact 58 without capture into thelight generating structure 56. The electron blocking layer 61 can beconfigured to be at least partially transparent to the light generatedby the light generating structure 56.

The various components of the heterostructure 50 can be formed from anysuitable materials, such as group III nitride materials as describedherein. In an embodiment, the n-type contact 54 is formed of a shortperiod superlattice that is at least partially transparent to radiationgenerated by the light generating structure 56, which can provide ahigher free hole concentration due to better dopant ionization, bettercrystal quality, and/or higher optical transmission to the emittedradiation. In a further embodiment, the n-type contact 54 (e.g., theshort period superlattice) is formed of group III nitride materials.

It is understood that additional layer(s) and/or structure(s) can beincluded in heterostructure 50. For example, the heterostructure 50 caninclude a reflective layer, a photonic crystal, a mirror, and/or thelike. These layer(s) and/or structure(s) can be configured to directlight generated by the light generating structure 56 in a manner thatincreases an amount of light emitted from heterostructure 50 than wouldbe emitted without the presence of the additional layer(s) and/orstructures. Similarly, one or more additional layers can be locatedbetween any of the layers shown in FIG. 6. For example, a buffer layerand/or a second layer can be formed directly on the substrate 52, andthe n-type contact 54 can be formed directly on the second layer.

In an embodiment, a heterostructure can include a light generatingstructure 56 located between a DBR structure 60 and a reflector, such asa metal reflector. In this case, the DBR structure 60 and the reflector(e.g. a reflective contact) can establish resonant optical fielddistribution, which can enhance an efficiency of light extraction fromthe heterostructure. The reflector can be formed of any type ofmaterial, which is at least partially reflective of the light generatedby the light generating structure 56. In an embodiment, the material ofthe reflector is selected according to its reflectivity of a range ofultraviolet light including a wavelength corresponding to the peakwavelength of ultraviolet light emitted by the light generatingstructure 56.

To this extent, FIG. 7 shows reflection coefficients of differentcoatings for illustrative reflective contacts. Illustrative reflectivecontacts can be formed from, among other things, aluminum, enhancedaluminum, aluminum silicon monoxide, aluminum magnesium fluoride,rhodium, enhanced rhodium, gold, and/or the like. As can be seen in FIG.7, rhodium and enhanced rhodium provide good reflectivity within theultraviolet range of wavelengths, particularly when compared to gold. Inparticular, enhanced rhodium provides excellent reflectivity in the deepultraviolet range of wavelengths (e.g., wavelengths below approximately0.3 micrometers (μm). However, rhodium does not provide good ohmiccontact to AlGaN materials.

In an embodiment, a light emitting diode, such as a deep ultravioletlight emitting diode, includes a composite reflecting contact. Forexample, FIG. 8A shows an illustrative configuration for an LED 62,which includes a composite contact 63 comprising a thin (e.g., 2-5nanometers thick) layer 64 of a first metal adjacent to a layer 66 ofrhodium and/or enhanced rhodium. Layer 64 can be formed of any metal,which is at least partially transparent to light generated by a lightemitting heterostructure 68 at the corresponding thickness of the layer64 and which provides improved ohmic contact and/or adhesion of thethicker reflective layer 66 to the surface of the heterostructure 68,such as a heterostructure formed of group III nitride materials. In anembodiment, layer 64 is formed of nickel (Ni). However, it is understoodthat layer 64 can be formed of any suitable material, including Nickeloxyhydroxide (NiOx), Palladium (Pd), Molybdenum (Mo), Cobalt (Co),and/or the like.

Various alternative composite contact configurations are possible. Forexample, FIG. 8B shows an illustrative configuration for an LED 70including a composite contact 72 formed of multiple layers of metals74A-74F (e.g., a metallic superlattice), each of which can be at leastpartially transparent or reflective of light emitted by a correspondinglight emitting heterostructure 76, such as a heterostructure formed ofgroup-III nitride materials, of the LED 70. In an embodiment, each ofthe layers of metals 74A-74F is configured to be at least partiallytransparent to the light emitted by the light emitting heterostructure76. For example, the layers of metals 74A-74F can include alternatingthin (e.g., 2-5 nanometers thick) layers of two metals selected from:Ni, NiOx, Pd, Mo, Co, and/or the like, which can be oxidized in an O₂ambient. Use of the multiple layers of metals 74A-74F can enableimproved reflectivity/transparency and/or polarization control of theradiation reflected by/passing through the composite contact 72. Whilethe composite contact 72 is shown including three repeating sets of twometals each, it is understood that the composite contact 72 can includeany combination of two or more metals and any number of layers.

In another embodiment, a composite contact can include graphene. Forexample, layer 64 of composite contact 63 (FIG. 8A) and/or a set oflayers 74A-74F of composite contact 72 can be formed of graphene, whichcan be configured to be transparent to light generated by thecorresponding heterostructure and very conductive. Another layer, suchas layer 66 of composite contact 63 and/or interlaced layers ofcomposite contact 72, can comprise a thin layer of metal adjacent to thegraphene, which can improve current spreading in the composite contact63, 72. In a further embodiment, the composite contact 63, 73 is atleast partially transparent to the light generated by theheterostructure. It is understood that an LED can include one or morelayers adjacent to a contact formed of graphene, which are configured toimprove light extraction from the LED, e.g., via a textured surface.

Furthermore, a composite contact of the light emitting diode can includeone or more non-uniform layers. For example, a non-uniform layer cancomprise a varying thickness and/or be absent from certain regions. FIG.8C shows an illustrative configuration for an LED 80, which includes acomposite contact 82 formed of a non-uniform transparent adhesion layer84 and a reflective layer 86. In an embodiment, the non-uniformtransparent adhesion layer 84 comprises nickel, the reflective layer 86comprises enhanced rhodium, and the light emitting heterostructure 88comprises a group III nitride heterostructure, which emits ultravioletradiation, such as deep ultraviolet radiation. In this case, theultraviolet radiation emitted by the light emitting heterostructure 88will not be partially absorbed by the transparent adhesion layer 84 inthe regions in which it is absent, thereby allowing for directreflection of the ultraviolet radiation by the reflective layer 86.

Additionally, the non-uniform distribution of the transparent adhesionlayer 84 can result in a non-uniform current, which is mostly limited tothe areas where the transparent adhesion layer 84 improves adhesion withthe surface of the light emitting heterostructure 88. As a result, acurrent density in these regions is higher than that for a uniformadhesion layer, which can thereby enhance radiative recombination.However, the configuration of the non-uniform transparent adhesion layer84 can be configured to limit the current non-uniformity to a range thatwill not result in local overheating within the LED 80, which couldresult in reliability problems for the LED 80.

A non-uniform transparent adhesion layer 84 can comprise any type ofdistribution along the surface of a light emitting heterostructure 88.For example, FIG. 8D shows an illustrative configuration for an LED 90,which includes a composite contact 92 formed of a non-uniformtransparent adhesion layer 94 and a reflective layer 96. In anembodiment, the non-uniform transparent adhesion layer 94 comprisesnickel, while the reflective layer 96 comprises enhanced rhodium, andthe light emitting heterostructure 88 comprises a group III nitrideheterostructure, which emits ultraviolet radiation, such as deepultraviolet radiation. In this case, the transparent adhesion layer 94is periodic, thereby forming a reflecting photonic crystal. Formation ofthe reflecting photonic crystal can improve the light reflection of thecomposite contact 92, and therefore the corresponding light extractionof light from the LED 90.

Sample transparent DUV LEDs were fabricated according to embodiments,along with conventional DUV LEDs for comparison. The DUV LEDs wereconfigured to emit radiation having a peak emission wavelength within orclose to the deep ultraviolet range. Each of the transparent DUV LEDsincluded a transparent Mg-doped AlGaN/AlGaN short period superlatticestructure (SPSL) as a cladding layer, which replaced transparent gradedp-type AlGaN cladding and p-type GaN contact layers of a typical LED.The DUV LED structures were grown on a sapphire substrate by acombination of metal-organic chemical vapor deposition (MOCVD) andmigration enhanced MOCVD. Each of the DUV LEDs included a thin p⁺⁺-GaNcontact layer to create a polarization induced high free holeconcentration near the surface and to improve the p-type contact. 300Kelvin (K) (e.g., room temperature) and 77 K Hall measurements for theDUV LEDs were taken, and indicated free hole concentration of 9.8×10¹⁷cm⁻³ and 9.6×10¹⁷ cm⁻³, respectively, which is consistent with theformation of a 2-dimensional (2D) hole gas. The measured hole mobilityincreased from 7.6 cm²/V s at 300 K to 11 cm²/V s at 77 K.

Optical transmission measurements of the DUV LEDs indicated up toapproximately eighty percent transmission at the peak LED emissionwavelength for the respective DUV LEDs. Furthermore, the Al-based andRh-based reflecting contacts provided more than sixty percentreflectivity within the deep ultraviolet range. FIG. 9 shows a chartcomparing illustrative transmission spectra of conventional andtransparent 340 nanometer DUV LEDs structures.

The forward voltage (V_(f)) of 340 nm DUV LEDs with conventional Ni/Aup-type contacts and absorbing and transparent p-type cladding layers wasmeasured to be 5.2 Volts (V) and 6.1 V at 20 mA, respectively. Use of areflecting p-type contact resulted in an additional approximately0.1-0.2 V increase of V_(f) due to the voltage drop across the contactbarrier. For shorter emission wavelengths, the voltage drop across SPSLcaused an increase in V_(f) from 5.3 V to 6.4 V. The output power oftransparent structure 330-340 nm emission LEDs with conventional andreflecting p-contacts were measured to be 0.83 mW and 0.91 mW at 20 mA,respectively. Devices from the reference wafer showed 0.36 mW at thesame current. Testing of 310 nm DUV LEDs before packaging showed asimilar increase in the DUV LED efficiency. To this extent, FIG. 10shows a chart illustrating an illustrative performance improvement of a340 nm DUV LED structure with a reflecting contact.

The heterostructure and/or contact designs described herein can beutilized in the formation of a device using a flip chip configuration.For example, FIG. 11 shows an illustrative configuration for a flip chipLED 100 according to an embodiment. In an embodiment, LED 100 cancomprise a deep ultraviolet LED, which is configured to emit radiationin the deep ultraviolet range of wavelengths. LED 100 can include amount 102, which is attached to a device heterostructure 104 using a setof bonding pads 106 and a set of solder bumps 108.

In an embodiment, the mount 102 is configured to provide protection forthe heterostructure 104 from transient voltage surges, such as thosecaused by electrostatic discharge (ESD), an electric power surge, and/orthe like. In a more particular embodiment, the mount 102 is formed of aslightly conductive material, which provides a parallel leakage path forthe device heterostructure 104. For example, the conductive material cancomprise a semi-insulating silicon carbide (SiC), which can comprise anyof various polytypes of SiC, such as 4H-SiC, 6H-SiC, 3C-SiC, high puritySiC, and/or the like. However, it is understood that the mount 102 cancomprise other types of conductive materials and/or ESD protectiveconfigurations.

As illustrated, the device heterostructure 104 can include, for example,a reflecting contact 110, a transparent adhesion layer 112 (which can beuniform or non-uniform as described herein), a p-type contact 114, ablocking layer 116, a light generating structure 118, and a n-typecontact 120. Each of the components of the heterostructure 104 can befabricated as described herein. During operation of the LED 100, thereflecting contact 110 can reflect light, such as ultraviolet light,emitted by the light generating structure 118 towards the n-type contact120. The n-type contact 120 can be at least partially transparent to thelight, thereby emitting the light from the LED 100. In an embodiment,the n-type contact 120 can comprise a textured surface 122, which isconfigured to improve extraction of the light from the LED 100.

The various heterostructures shown and described herein can beimplemented as part of various types of devices, such as a lightemitting diode (LED), a superluminescent diode, a laser, and/or thelike. In an embodiment, the device is configured to emit ultravioletradiation during operation (e.g., an ultraviolet LED, an ultravioletsuperluminescent LED, and/or the like). In a more particular embodiment,the ultraviolet radiation comprises deep ultraviolet radiation, e.g.,210 nm to 365 nm.

As used herein, a layer is at least partially transparent when the layerallows at least a portion of light in a corresponding range of radiationwavelengths to pass there through. For example, a layer can beconfigured to be at least partially transparent to a range of radiationwavelengths corresponding to a peak emission wavelength for the light(such as ultraviolet light or deep ultraviolet light) emitted by a lightgenerating structure described herein (e.g., peak emission wavelength+/− five nanometers). As used herein, a layer is at least partiallytransparent to radiation if it allows more than approximately 0.001percent of the radiation to pass there through. In a more particularembodiment, an at least partially transparent layer is configured toallow more than approximately five percent of the radiation to passthere through. Similarly, a layer is at least partially reflective whenthe layer reflects at least a portion of the relevant light (e.g., lighthaving wavelengths close to the peak emission of the light generatingstructure). In an embodiment, an at least partially reflective layer isconfigured to reflect more than approximately five percent of theradiation.

In an embodiment, a structure described herein can include one or morelayers having a composition selected such that the layer has atransparency of at least a target transparency to radiation, such asultraviolet radiation, of a target set of wavelengths. For example, alayer can be a group III nitride-based layer, such as an electronblocking layer or a p-type contact layer described herein, which iscomposed of Al_(x)Ga_(1-x)N where the aluminum molar fraction (x) issufficiently high in some domains of the layer to result in the layerbeing at least partially transparent to ultraviolet radiation. In anembodiment, the layer can comprise a superlattice layer located in anemitting device configured to emit radiation having a dominantwavelength in the ultraviolet spectrum, and the composition of at leastone sub-layer in each period of the superlattice layer is configured tobe at least partially transparent to ultraviolet radiation having atarget wavelength corresponding to the ultraviolet radiation emitted bythe emitting device.

An amount of transparency of a short period superlattice (SPSL) can beapproximated by computing an averaged band gap of the SPSL, and deducingaverage absorption coefficients of the SPSL. The absorption coefficientsdepend on an absorption edge of the semiconductor material, which formaterials formed of an AlGaN alloy, is a function of the molar fractionsof the Al_(x)Ga_(1-x)N semiconductor alloy.

In an embodiment, the target transparency for the material is at leastten times more transparent than the least transparent layer of materialin the structure (e.g., GaN for a group III nitride-based device). Inthis case, an absorption coefficient of the semiconductor layer can beon the order of 10⁴ inverse centimeters or lower. In this case, a onemicron thick semiconductor layer will allow approximately thirty-sixpercent of the ultraviolet radiation to pass there through.

FIG. 12 shows a dependence of the absorption coefficient on thewavelength for various aluminum molar fractions (x) of anAl_(x)Ga_(1-x)N alloy according to an embodiment. In order to maintainan absorption coefficient of the semiconductor layer at orders of 10⁴inverse centimeters or lower, the content of aluminum in an SPSL barrierlayer can be chosen based on the corresponding target wavelength orrange of wavelengths. For example, for a target wavelength ofapproximately 250 nanometers, the aluminum molar fraction can beapproximately 0.7 or higher, whereas for a target wavelength ofapproximately 300 nanometers, the aluminum molar fraction can be as lowas approximately 0.4. FIG. 13 shows an illustrative chart for selectingan aluminum content of an Al_(x)Ga_(1-x)N alloy to maintain a targettransparency for a corresponding emitted wavelength, λ, according to anembodiment. In this case, the target transparency corresponds to anabsorption coefficient of the semiconductor layer on the order of 10⁴inverse centimeters. Note that in FIG. 13, the dependence of x=x(λ) islinear, with x=C·λ+B, where C=−0.0048 [1/nm], and B=1.83.

In an embodiment, a device can include one or more layers with lateralregions configured to facilitate the transmission of radiation throughthe layer and lateral regions configured to facilitate current flowthrough the layer. For example, the layer can be a short periodsuperlattice, which includes barriers alternating with wells. In thiscase, the barriers can include both transparent regions, which areconfigured to reduce an amount of radiation that is absorbed in thelayer, and higher conductive regions, which are configured to keep thevoltage drop across the layer within a desired range. As used herein,the term lateral means the plane of the layer that is substantiallyparallel with the surface of the layer adjacent to another layer of thedevice. As described herein, the lateral cross section of the layer caninclude a set of transparent regions, which correspond to those regionshaving a relatively high aluminum content, and a set of higherconductive regions, which correspond to those regions having arelatively low aluminum content.

The set of transparent regions can be configured to allow a significantamount of the radiation to pass through the layer, while the set ofhigher conductive regions can be configured to keep the voltage dropacross the layer within a desired range (e.g., less than ten percent ofa total voltage drop across the structure). In an embodiment, the set oftransparent regions occupy at least ten percent of the lateral area ofthe layer, while the set of higher conductive regions occupy at leastapproximately two percent (five percent in a more specific embodiment)of the lateral area of the layer. Furthermore, in an embodiment, a bandgap of the higher conductive regions is at least five percent smallerthan the band gap of the transparent regions. In a more particularembodiment, the transparent regions comprise a transmission coefficientfor radiation of a target wavelength higher than approximately sixtypercent (eighty percent in a still more particular embodiment), whilethe higher conductive regions have a resistance per unit area tovertical current flow that is smaller than approximately 10⁻² ohm·cm².As used herein, the term transmission coefficient means the ratio of anamount of radiation exiting the region to an amount of radiationentering the region.

The transparent and conductive regions can be formed using any solution.For example, a layer can be grown using migration-enhanced metalorganicchemical vapor deposition (MEMOCVD). During the growth, inhomogeneitiesin the lateral direction of a molar fraction of one or more elements,such as aluminum, gallium, indium, boron, and/or the like, can beallowed in the layer. In an embodiment, such compositionalinhomogeneities can vary by at least one percent.

While shown and described herein as a method of designing and/orfabricating a structure and/or a corresponding semiconductor deviceincluding the structure, it is understood that aspects of the inventionfurther provide various alternative embodiments. For example, in oneembodiment, the invention provides a method of designing and/orfabricating a circuit that includes one or more of the devices designedand fabricated as described herein.

To this extent, FIG. 14 shows an illustrative flow diagram forfabricating a circuit 146 according to an embodiment. Initially, a usercan utilize a device design system 130 to generate a device design 132using a method described herein. The device design 132 can compriseprogram code, which can be used by a device fabrication system 134 togenerate a set of physical devices 136 according to the features definedby the device design 132. Similarly, the device design 132 can beprovided to a circuit design system 140 (e.g., as an available componentfor use in circuits), which a user can utilize to generate a circuitdesign 142 (e.g., by connecting one or more inputs and outputs tovarious devices included in a circuit). The circuit design 142 cancomprise program code that includes a device designed using a methoddescribed herein. In any event, the circuit design 142 and/or one ormore physical devices 136 can be provided to a circuit fabricationsystem 144, which can generate a physical circuit 146 according to thecircuit design 142. The physical circuit 146 can include one or moredevices 136 designed using a method described herein.

In another embodiment, the invention provides a device design system 130for designing and/or a device fabrication system 134 for fabricating asemiconductor device 136 using a method described herein. In this case,the system 130, 134 can comprise a general purpose computing device,which is programmed to implement a method of designing and/orfabricating the semiconductor device 136 as described herein. Similarly,an embodiment of the invention provides a circuit design system 140 fordesigning and/or a circuit fabrication system 144 for fabricating acircuit 146 that includes at least one device 136 designed and/orfabricated using a method described herein. In this case, the system140, 144 can comprise a general purpose computing device, which isprogrammed to implement a method of designing and/or fabricating thecircuit 146 including at least one semiconductor device 136 as describedherein.

The foregoing description of various aspects of the invention has beenpresented for purposes of illustration and description. It is notintended to be exhaustive or to limit the invention to the precise formdisclosed, and obviously, many modifications and variations arepossible. Such modifications and variations that may be apparent to anindividual in the art are included within the scope of the invention asdefined by the accompanying claims.

What is claimed is:
 1. A light emitting heterostructure comprising: ann-type contact layer formed of a group III nitride material; and a lightgenerating structure adjacent to the n-type contact layer, the lightgenerating structure formed within a group III nitride material havingan aluminum molar fraction greater than fifty percent and forming aconduction band energy tub with respect to the n-type contact layer andincluding a plurality of quantum wells, wherein a difference between aconduction band edge energy of the n-type contact layer and an electronground state energy of a quantum well in the plurality of quantum wellsis greater than an energy of a polar optical phonon in a material of thelight generating structure, and wherein a width of the light generatingstructure is comparable to a mean free path for emission of a polaroptical phonon by an electron injected into the material forming theconduction band energy tub.
 2. The heterostructure of claim 1, whereinthe width of the light generating structure exceeds the mean free pathfor emission of a polar optical phonon by an electron injected into thematerial forming the conduction band energy tub by less thanapproximately ten percent, and wherein the difference exceeds the energyof the polar optical phonon by approximately thermal energy.
 3. Theheterostructure of claim 1, further comprising a blocking layer locatedon an opposite side of the light generating structure as the n-typecontact layer, wherein a difference between a conduction band edgeenergy of the blocking layer and the electron ground state energy of aquantum well in the plurality of quantum wells is greater than theenergy of a polar optical phonon in the material of the light generatingstructure.
 4. The heterostructure of claim 1, wherein the lightgenerating structure further includes a set of barriers alternating withthe plurality of quantum wells, the set of barriers including a firstbarrier immediately adjacent to a first quantum well in the plurality ofquantum wells and the n-type contact layer, wherein a thickness of thefirst barrier is sufficient to accelerate an injected electron in anelectric field to reach the energy of the polar optical phonon withrespect to the electron ground state energy in the first quantum well,and wherein a thickness of a remainder of the light generating structureexceeds the mean free path for emission of the polar optical phonon. 5.The heterostructure of claim 1, further comprising: an at leastpartially transparent substrate, wherein the n-type contact is locatedbetween the substrate and the light generating structure, and whereinthe at least partially transparent substrate and the n-type contactlayer are at least partially transparent to the light generated by thelight generating structure; and a Bragg reflector structure formed on anopposite side of the light generating structure as the n-type contact,wherein the Bragg reflector structure is configured to reflect lightgenerated by the light generating structure towards the substrate. 6.The heterostructure of claim 5, further comprising an electron blockinglayer formed between the light generating structure and the Braggreflector structure.
 7. The heterostructure of claim 5, wherein then-type contact comprises a short period superlattice.
 8. Theheterostructure of claim 1, further comprising a composite contactlocated on an opposite side of the light generating structure as then-type contact layer, the composite contact comprising: an adhesionlayer, wherein the adhesion layer is at least partially transparent tolight generated by the light generating structure; and a reflectingmetal layer configured to reflect at least a portion of the lightgenerated by the light generating structure.
 9. The heterostructure ofclaim 8, wherein the adhesion layer comprises a thin layer of one of:nickel, palladium, molybdenum, or cobalt, and wherein the reflectingmetal layer comprises one of: aluminum, enhanced aluminum, rhodium, orenhanced rhodium.
 10. The heterostructure of claim 8, wherein theadhesion layer comprises a non-uniform layer.
 11. The heterostructure ofclaim 10, wherein the non-uniform adhesion layer is configured to form aphotonic crystal.
 12. The heterostructure of claim 8, further comprisinga distributed semiconductor heterostructure Bragg reflector (DBR)structure, wherein the DBR structure is located on an opposite side ofthe light generating structure as the composite contact.
 13. Theheterostructure of claim 1, further comprising a composite contactlocated on an opposite side of the light generating structure as then-type contact layer, wherein at least one layer of the compositecontact is formed of graphene.
 14. The heterostructure of claim 1,further comprising a metallic superlattice contact located on anopposite side of the light generating structure as the n-type contactlayer, wherein the metallic superlattice includes a plurality ofintertwined layers of a first metal and a second metal distinct from thefirst metal, and wherein the metallic superlattice is at least partiallytransparent to light generated by the light generating structure.
 15. Alight emitting heterostructure comprising: an n-type contact layer; anda light generating structure adjacent to the n-type contact layer, thelight generating structure formed within a material forming a conductionband energy tub with respect to the n-type contact layer and including aplurality of quantum wells, wherein a width of the light generatingstructure is comparable to a mean free path for emission of a polaroptical phonon by an electron injected into the material forming theconduction band energy tub; and a blocking layer located on an oppositeside of the light generating structure as the n-type contact layer,wherein a difference between a conduction band edge energy of theblocking layer and an electron ground state energy of a quantum well inthe plurality of quantum wells is greater than an energy of a polaroptical phonon in a material of the material forming the conduction bandenergy tub.
 16. The heterostructure of claim 15, wherein the width ofthe light generating structure exceeds the mean free path for emissionof a polar optical phonon by an electron injected into the materialforming the conduction band energy tub by less than approximately tenpercent.
 17. The heterostructure of claim 15, wherein the n-type contactlayer is formed of a group III nitride material, and wherein thematerial forming the conduction band energy tub is a group III nitridematerial having an aluminum molar fraction greater than fifty percent.18. The heterostructure of claim 15, wherein the light generatingstructure further includes a set of barriers alternating with theplurality of quantum wells, the set of barriers including a firstbarrier immediately adjacent to a first quantum well in the plurality ofquantum wells and the n-type contact layer, wherein a thickness of thefirst barrier is sufficient to accelerate an injected electron in anelectric field to reach the energy of the polar optical phonon withrespect to the electron ground state energy in the first quantum well,and wherein a thickness of a remainder of the light generating structureexceeds the mean free path for emission of the polar optical phonon. 19.A light emitting device comprising: an n-type contact layer; a lightgenerating structure adjacent to the n-type contact layer, wherein thelight generating structure is formed within a material forming aconduction band energy tub with respect to the n-type contact layer, thelight generating structure including: a set of quantum wells, wherein adifference between a conduction band edge energy of the n-type contactlayer and an electron ground state energy of a quantum well in the setof quantum wells is greater than an energy of a polar optical phonon inthe material forming the conduction band energy tub; and a set ofbarriers alternating with set of quantum wells, the set of barriersincluding a first barrier immediately adjacent to a first quantum wellin the set of quantum wells and the first side of the light generatingstructure, wherein a thickness of the first barrier is sufficient toaccelerate an injected electron in an electric field to reach the energyof the polar optical phonon with respect to the electron ground stateenergy in the first quantum well, and wherein a thickness of a remainderof the light generating structure exceeds a mean free path for emissionof a polar optical phonon by an electron injected into the materialforming the conduction band energy tub; and a composite contact locatedon an opposite side of the light generating structure as the n-typecontact layer, the composite contact comprising: an adhesion layer,wherein a material forming the adhesion layer has a maximum thicknessless than five nanometers and allows at least five percent of the lightgenerated by the light generating structure to pass there through; and areflecting metal layer configured to reflect at least a portion of thelight generated by the light generating structure.
 20. The device ofclaim 19, wherein the n-type contact layer is formed of a group IIInitride material, and wherein the material forming the conduction bandenergy tub is a group III nitride material having an aluminum molarfraction greater than fifty percent.